Senescent changes in human vision have been well documented, but the factors responsible for these changes are only partially understood. The proposed research is intended to provide fundamental data on aging of the human visual system using psychophysical methods that separate the relative contributions of optical and neural mechanisms. Because much of the sensitivity loss of cone pathways occurs at early stages of processing, we will examine age-related losses in cone photopigment optical density in the central 10 degree of retina for normal and sex-linked dichromats whose genes have been sequenced to look for genetic correlates of photopigment optical density and receptor "longevity." It is hypothesized that senescent changes in cone photopigment density will not be uniform with retinal eccentricity, and the outcome of this experiment will be important for theoretical and practical reasons. A second aim of the proposed research is to quantify senescent changes in the temporal properties of isolated color mechanisms through measurements of the temporal contrast sensitivity function and the impulse response function. The third aim is to determine senescent changes in spatial properties of isolated color mechanisms through measurements of the area of complete spatial summation (Ricco's area). Age-related changes in spatial summation are predicted based on our previous work, and documented losses of retinal ganglion cells with age, but we do not know whether this will be found in all color pathways. The fourth aim is also concerned with spatial properties of rod and cone pathways, but will measure spatial contrast sensitivity functions for isolated mechanisms. The final aim is based on our previous work showing that, at a perceptual level, the visual system employs mechanisms that compensate, in part, for losses in sensitivity at lower levels. Color appearance mechanisms will be proved by measurements of age-related changes in chromatic perceptive fields over areas of retina associated with different relative losses of cones and ganglion cells. Sensitivity of isolated cone mechanisms and color appearance will be measured for individuals before and after cataract extraction and implantation of intra-ocular lenses. These experiments will provide new information on mechanisms that promote color constancy despite substantial variations in the retinal image due to lenticular senescence. Each of these alms will include detailed tests of individual younger and older observers, and measurement on selected conditions with a large group of individuals ranging in age from approximately 12 to 85 years. In addition to providing basic data on the aging visual system, the experiments will provide probes for models of how the visual system adapts and compensates for degradations in the optical and neural images that occur with senescence and disease.